Car not starting and probably 12v

If you are not driving your car week on week, starting it for only 15 minutes once a week is likely to deplete your battery faster. You need at a bare minimum to start it for one hour once per week. One hour twice a week would be better. For your current situation, you'd be advised to fully charge your 12 V battery with a 5 Amp battery charger.

Yes, sure there would be no problem doing this, but if you charged it for longer, you wouldn't need a mechanic visit.

Is there a reason you're not using a battery charger (a fully automatic multi-stage one) while the car is not driven?

 

If your car is parked outside you can piece together a small solar panel and charge controller. Very cheap and easy you can get the parts at harbor freight.
I used this on a class c RV that was stored for almost a year and no battery drain.

First
You'll need to test your battery to make sure it's good, most auto parts places will charge it fully and load test it, but you will need to take it out.



30 Amp Solar Charge Controller

Solar Power Connection Cable Kit

1.5 Watt Solar Battery Maintainer

Pixel 7 Pro ?

 

Probably time to give up your car or move back to a Yaris or Corolla that just requires gas The savings aren't helping you it sounds like

 

Definitely get a charger, a smart charger, 3~4 amp rating. Having a fuel efficient car and driving it purely to charge the battery doesn't sound right, does it...

We're in the same boat: last drive was eight days ago. Our car LIVES on a charger, pretty much constantly between uses: a CTEK 4.3 (current iteration is CTEK 5). I've wired in the CTEK supplied quick-connect cable, makes hook up easy. Our battery was 8 years installed as of September, still tests like new.

How old is the battery? I'd suspect it's toast, with that history. You could do a simple voltage test first (with a digital multimeter, $20~ for a cheap one), and post here. Test directly at the battery posts.

For a better test something like Solar BA9 (an electronic load tester, one of many) will diagnose the battery, determine it's viability.

Or you can bring the loose battery into most automotive retailers, ones that sell batteries, have them test it for free. Phone in advance: it would be good find a place that has your specific battery in stock, makes it one-stop shopping if the current battery proves to be dead (which it likely will).

 

Even an old age until I'm hoveround bound or something I can come up with a way to drive the car 16 miles a day or some such nonsense It's fun to do something else to do all that sort of thing so until I medically unable everything seems to be reasonable.